A technique is known in the prior art in which a three-dimensionally shaped object is manufactured by irradiating a material layer formed of a metallic material with light beams (directional energy beams, for example, laser beams) to form a sintered layer or molten layer, and repeatedly performing the process of forming another material layer on the sintered layer or molten layer and irradiating it with light beams to form a sintered layer or molten layer. This technique has an advantage of enabling a complex three-dimensional object to be manufactured in a short time. If light beams with high energy density are used, the metallic material can be almost completely melted before being solidified. That is, after the melting, the material can be almost 100 percent dense. By finishing the surface of the dense shaped object, the finished object can have a smooth surface so as to be applied to a plastic mold or the like.
This technique called metal laser sintering typically uses a metallic material in powder form. Using metal powder as the material to be applied in layers can increase the surface area of the material and enhance the absorption of laser light. This enables the material to be sintered or melted even under conditions of low energy density irradiation, reducing the time required for shaping an object.
In order to obtain a shaped object having adequate strength, it is necessary not only to improve the strength of adhesion to a portion adjacent to a laser applied portion in a material layer irradiated with laser beams but also to improve the strength of adhesion between an irradiated portion and an underlying layer. When the material to be applied in layers is metal powder, laser light passes through the spaces between particles to reach the underlying layer. Thereby, the underlying layer is heated so that the adhesion strength can be improved.
Further, the upper surface of a portion irradiated with laser should not be raised so much. When another material layer is formed to shape a subsequent layer, if the swelling height is greater than the thickness of a layer of the metal powder, the formation itself of the material layer would be difficult.
Of course, the outside of a shaped object must not be cracked. It is also preferable to eliminate any micro crack in the internal structure.
A metallic material irradiated with laser is once melted partially or completely and then cooled and solidified into a shaped object. In the molten state, when the wettability is high, the area joined to an adjacent shaped portion becomes larger, and when the fluidity is high, the swelling becomes less. Therefore, higher fluidity and higher wettability in the molten state are desired.
In view of that, the applicant of this application has proposed a powder mixture for metal laser sintering that comprises iron group metal powder of chrome molybdenum steel, nickel powder, and phosphor copper or manganese copper powder. The chrome molybdenum steel powder is adopted due to its hardness and strength, the nickel powder is adopted due to its strength toughness, and workability, and the phosphor copper or manganese copper powder is adopted due to its wettability and fluidity.
It is difficult to manufacture a dense three-dimensionally shaped object by irradiating only iron group metal powder with laser. This is because it is difficult to join a subsequent layer to a previously formed layer of iron with no gap therebetween. Although chrome molybdenum steel itself is hard and excellent in mechanical strength, a three-dimensionally shaped object obtained by laser irradiation of chrome molybdenum steel powder alone is less dense and low in strength.
When the iron group metal powder is an alloy containing a high proportion of nickel component, the above mentioned problem is significant because a hard oxide film formed on the surface of the powder interferes with fusion of particles of the iron group metal powder. The inclusion of nickel in iron group metal has the advantage of being able to improve the toughness, strength, and corrosion resistance of the iron group metal. However, in the case where it is used for manufacturing a three-dimensionally shaped object by laser irradiation, the advantage is not utilized at all.
If a high energy laser is used, even iron group metal powder containing chrome molybdenum steel and a nickel component can be fused adequately. However, this has the drawback of requiring a large-scale laser device and requiring much power, thus increasing the manufacturing cost. Besides, since the laser scanning speed cannot be increased, the efficiency in manufacturing is decreased. Further, a shaped object formed with an excessive amount of irradiation energy is liable to be warped or deformed due to thermal stress.
Copper is a metallic material that is excellent in fluidity when melted, is excellent in wettability to an iron group material in the molten state, and hardly deteriorates in characteristics even when alloyed with an iron group material. When a powder mixture of iron group metal powder and copper alloy powder is irradiated with laser, the copper alloy is first melted to fill the spaces between particles of the iron group metal powder, and at the same time, this serves as a bonding material for fusion. In the case where a high energy laser is used, iron powder and copper alloy powder, which form a powder mixture, are totally melted into an alloy.
The fluidity of molten metal increases as the difference between the temperature in the molten state and the melting point increases. Phosphor copper alloy and manganese copper alloy have lower melting points than pure copper, and thus have higher fluidity than pure copper when they are irradiated with the same energy.
A conventional iron powder material for metal laser sintering contains nickel powder. As described above, when an alloy containing a nickel component is used as iron group metal powder, a hard oxide film formed on the surface of the powder interferes with fusion of the particles of the powder. However, in the case where nickel powder is mixed with a copper alloy separately from iron group metal powder, particles of these powders can be fused well. A hardened layer comprising an iron component, nickel, and a copper alloy component has a high sintered density, thus being tough and strong.
Especially, when 60 to 90 wt % of chrome molybdenum steel, 5 to 35 wt % of nickel powder, and 5 to 15 wt % of copper manganese alloy powder are contained, particularly desirable results can be obtained.
In metal laser sintering using the above mentioned metal powder material, generally desirable results have been obtained in that a complex three-dimensionally shaped object can be achieved by laser irradiation and formation of the layers.
However, a metal powder material cannot have a high filling density when the powder is applied uniformly in a thin layer. Therefore, unless the energy density of the laser radiation is increased, a dense shaped object cannot be obtained and thus the strength of a shaped object is low.
Further, when powder is melted or sintered by laser irradiation, the more space between the particles, the more shrinkage will occur. It remains as internal stress in the shaped object, and thus causes the shaped object to be liable to deformation and warpage and to be low in dimensional accuracy.